linux-kernel - Re: [PATCH] PCI: dwc: Use multiple ATU regions for large bridge windows

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Message-ID: <aPDObXsvMoz1OYso@ryzen>
Date: Thu, 16 Oct 2025 12:52:29 +0200
From: Niklas Cassel <cassel@...nel.org>
To: Samuel Holland <samuel.holland@...ive.com>
Cc: Jingoo Han <jingoohan1@...il.com>,
	Manivannan Sadhasivam <mani@...nel.org>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Krzysztof Wilczyński <kwilczynski@...nel.org>,
	Rob Herring <robh@...nel.org>, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH] PCI: dwc: Use multiple ATU regions for large bridge
 windows

On Wed, Oct 15, 2025 at 04:15:01PM -0700, Samuel Holland wrote:
> Some SoCs may allocate more address space for a bridge window than can
> be covered by a single ATU region. Allow using a larger bridge window
> by allocating multiple adjacent ATU regions.
> 
> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> ---
> An example of where this is needed is the ESWIN EIC7700 SoC[1]. The SoC
> decodes 128 GiB of address space to the PCIe controller. Without this
> change, only 8 GiB is usable; after this change 48 GiB (6 ATU regions)
> is usable, which allows using PCIe cards with >8 GiB BARs:
> 
> eic7700-pcie 54000000.pcie: host bridge /soc/pcie@...00000 ranges:
> eic7700-pcie 54000000.pcie:       IO 0x0040800000..0x0040ffffff -> 0x0040800000
> eic7700-pcie 54000000.pcie:      MEM 0x0041000000..0x004fffffff -> 0x0041000000
> eic7700-pcie 54000000.pcie:      MEM 0x8000000000..0x89ffffffff -> 0x8000000000
> eic7700-pcie 54000000.pcie: iATU: unroll T, 8 ob, 4 ib, align 4K, limit 8G
> eic7700-pcie 54000000.pcie: PCIe Gen.2 x1 link up
> eic7700-pcie 54000000.pcie: PCI host bridge to bus 0000:00
> 
> [1]: https://lore.kernel.org/linux-pci/20250923120946.1218-1-zhangsenchuan@eswincomputing.com/
> 
>  .../pci/controller/dwc/pcie-designware-host.c | 34 ++++++++++++-------
>  1 file changed, 22 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> index 20c9333bcb1c..148076331d7b 100644
> --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> @@ -873,30 +873,40 @@ static int dw_pcie_iatu_setup(struct dw_pcie_rp *pp)
>  
>  	i = 0;
>  	resource_list_for_each_entry(entry, &pp->bridge->windows) {
> +		u64 total_size;
> +
>  		if (resource_type(entry->res) != IORESOURCE_MEM)
>  			continue;
>  
> -		if (pci->num_ob_windows <= ++i)
> -			break;
> -
> -		atu.index = i;
>  		atu.type = PCIE_ATU_TYPE_MEM;
>  		atu.parent_bus_addr = entry->res->start - pci->parent_bus_offset;
>  		atu.pci_addr = entry->res->start - entry->offset;
>  
>  		/* Adjust iATU size if MSG TLP region was allocated before */
>  		if (pp->msg_res && pp->msg_res->parent == entry->res)
> -			atu.size = resource_size(entry->res) -
> +			total_size = resource_size(entry->res) -
>  					resource_size(pp->msg_res);
>  		else
> -			atu.size = resource_size(entry->res);
> +			total_size = resource_size(entry->res);
>  
> -		ret = dw_pcie_prog_outbound_atu(pci, &atu);
> -		if (ret) {
> -			dev_err(pci->dev, "Failed to set MEM range %pr\n",
> -				entry->res);
> -			return ret;
> -		}
> +		do {
> +			if (pci->num_ob_windows <= ++i)
> +				break;
> +
> +			atu.index = i;
> +			atu.size = min(total_size, pci->region_limit + 1);
> +
> +			ret = dw_pcie_prog_outbound_atu(pci, &atu);
> +			if (ret) {
> +				dev_err(pci->dev, "Failed to set MEM range %pr\n",
> +					entry->res);
> +				return ret;
> +			}
> +
> +			atu.parent_bus_addr += atu.size;
> +			atu.pci_addr += atu.size;
> +			total_size -= atu.size;

1) Personal opinion, but perhaps:

			total_size -= atu.size;
			if (total_size) {
				atu.parent_bus_addr += atu.size;
				atu.pci_addr += atu.size;
			}

To more clearly show that these lines are performed/relevant only if
total_size > atu_limit ?


2) Perhaps convert the loop from:

do {

} while (total_size);


To a:

while (total_size) {

}

I don't see any reason for this to be a do-while loop.

while () also has the benefit that you will not "waste" an outbound iatu
if you get a window of size 0.


3) Since you are modifying the outbound range to support sizes > atu_limit,
I think you should do the same change for dma-ranges (inbound translation):

https://github.com/torvalds/linux/blob/v6.18-rc1/drivers/pci/controller/dwc/pcie-designware-host.c#L928-L948

Because it feels inconsistent if we allow size > atu_limit for outbound,
but not for inbound translation.


Kind regards,
Niklas